The huge increases in migrants over the last decade were partly due to a politically motivated attempt by ministers to radically change the country and “rub the Right’s nose in diversity”, according to Andrew Neather, a former adviser to Tony Blair, Jack Straw and David Blunkett.

He said Labour’s relaxation of controls was a deliberate plan to “open up the UK to mass migration” but that ministers were nervous and reluctant to discuss such a move publicly for fear it would alienate its “core working class vote”.

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Writing in the Evening Standard, he revealed the “major shift” in immigration policy came after the publication of a policy paper from the Performance and Innovation Unit, a Downing Street think tank based in the Cabinet Office, in 2001.

He wrote a major speech for Barbara Roche, the then immigration minister, in 2000, which was largely based on drafts of the report.

He said the final published version of the report promoted the labour market case for immigration but unpublished versions contained additional reasons, he said.

He wrote: “Earlier drafts I saw also included a driving political purpose: that mass immigration was the way that the Government was going to make the UK truly multicultural.

“I remember coming away from some discussions with the clear sense that the policy was intended – even if this wasn’t its main purpose – to rub the Right’s nose in diversity and render their arguments out of date.”